The Fork Fire was reported around 3:30 p.m. in the area of Glendora Mountain and East Fork roads. It was initially estimated at about five to 10 acres, but by 4 p.m., it was reported at 75 acres and went to a second alarm as more ground and air resources were dispatched to the scene.
As of Saturday morning, the fire has scorched 330 acres, according to fire officials.
East Fork Road was closed from Highway 39 to Glendora Mountain Road.
The National Weather Service reported temperatures in the mid-80s near the fire with relatively light winds gusting to 12 mph.
It's unclear what sparked the blaze.
Fire officials said that a night-flying firefighting helicopter will be working throughout the night.
City News Service contributed to this report.